This invention relates to electrical power generating systems that include synchronous dynamolectric machines and more particularly to winding configurations and system components which permit such machines to operate as either a starter or a generator in combination with a separate prime mover.
In airborne electrical power generation systems, it is desirable to have a single system which provides both the starter and generator functions. At present, aircraft engines are generally started by motor starters or gas turbines which are dedicated machines. In order to reduce cost and weight, it would be desirable to use the aircraft generators as motors during the engine start-up period and thereby eliminate the machines which were dedicated to engine starting.
Variable speed constant frequency (VSCF) power generation systems are commonly used for aircraft applications. In these systems, the aircraft engine drives a generator at a variable speed, thereby producing a variable frequency generator output. This output is then converted to a constant frequency output. In one type of VSCF system, the output of the variable speed generator is rectified to supply DC power to a pair of DC link conductors. An inverter circuit receives DC power from the link conductors and produces a constant frequency AC output. The inherent simplicity and reliability of DC link VSCF systems has been established and it is desired to modify the exiting designs to provide starter capability. To use such a system for engine starting, the converter (rectifier-inverter combination) could be reversed such tha ground power or another on-board aircraft electrical system would supply multiple phase constant frequency power to the converter. The converter would then supply the required variable frequency, multiple phase power to the main terminals of the generator to operate the generator as a motor, thereby driving the engine up to the desired speed. After the engine ignites and reaches idling speed, the synchronous machine is reconnected and operates as a generator to provide electrical power for the aircraft.
Operating a synchronous dynamoelectric machine as a motor with the subsequent conversion to generator operation can introduce a number of problems with respect to excitation of the machine. For initial motor operation, the main rotating field of the machine must be excited to provide flux even at zero rpm. This flux interacts with the main stator winding current to supply torque. The conventional aircraft generator has rotating field winding. For the generator to act as a motor and supply full torque which is capable of starting an aircraft engine from zero rpm, it is necessary that the rotating field be fully excited from zero rpm and up. At zero rpm, only transformer action is capable of supplying excitation to the rotating field. Numerous systems which use a dynamoelectric machine as both a starter and a generator have been designed and examples of such systems are disclosed in U.S. Pat. Nos. 3,809,914; 3,908,130; 3,908,161; 3,937,97; 4,093,869; 4,330,743 and 4,481,459. However, it is still desirable to have a synchronous dynamoelectric machine which includes a particular exciter stator winding configuration such that the machine may be operated as both a starter and a generator in conjunction with a DC link VSCF power system with only minor modifications of the existing VSCF system hardware.